Injection molding method

ABSTRACT

An injection molding method for forming molded products of foamed plastics, in which molten resin mixed with foaming agent which begins foaming after being kept at a required temperature for a required time is injected into a metal mold wherein the foaming agent mixed together with the molten resin is solidified in a body in the metal mold under such conditions that the portion of the foaming agent molten resin mixture which is in contact with the inner surface of the metal mold is accordingly cooled rapidly such as to substantially prevent foaming thereof, so that the appearance of the molded product is finished neatly, while the foaming agent/molten resin mixture in the inner part of the mold is cooled gradually so that the temperature thereof is not rapidly decreased, under conditions wherein the foaming agent begins foaming and the inner portion of the molten resin is therefore expanded from inside owing to the foaming pressure, which expansion substantially prevents the occurrence of depressions in the surface of the molded product which might otherwise be caused due to shrinkage upon cooling of the resin.

DESCRIPTION TECHNICAL FIELD

This invention relates to an injection molding method for forming moldedproducts of foamed plastics.

BACKGROUND ART

Conventionally, an injection molding machine is known which comprises aninjection mechanism for melting and injecting a material to be formed byinjection molding such as, for example, a thermoplastic material or athermosetting resin (referred to simply as "resin", hereinafter) bymeans of a heating cylinder which is provided with a screw (and/or aplunger), a mold clamping mechanism for clamping a metal mold with ahigh pressure, and a control mechanism for automatically operating thesemechanisms, and which operates in such a manner that the resin isdropped from a hopper into the heating cylinder to be melted and mixedand kneaded, the resin thus heated and molten is forced (injected) intothe metal mold at a high pressure by the forward movement of the screwand, after being cooled and solidified, is then released from the mold,thereby obtaining molded products.

When the resin which is melted by heating is injected into the metalmold, a part of the molten resin existing close to a surface of themetal mold (that is, a layer of resin which forms an outer portion of amolded product) 1 is cooled rapidly, while another part of the moltenresin existing in the inner part of the metal mold (that is, a layer ofresin which forms an inner part of the molded product) 2 is cooledgradually, as shown in FIG. 4, thereby causing shrinkage of the resin.

On the other hand, as shown in FIG. 5, the injection pressure 3 ismaximized immediately after the commencement of the injection anddecreases sharply upon the completion of filling of the mold with theresin (or when changed over to a hold pressure). To the contrary, theinternal pressure 4 in the metal mold becomes sharply reduced when themolded product is taken out (or when the cooling is completed) althoughit is very high immediately after the completion of the filling of themold with the resin. In short, the molten resin is subjected to verycomplicated variations in operations of parameters in that it contractsdue to the compressing action of high injection pressure appliedimmediately after the commencement of the injection and it expands dueto the decompressing action of the respective pressure mentioned above.

In this way, the molten resin suffers a difference in volume caused dueto its complicated behavior mentioned above, or what is called "moldingshrinkage", in the course of the injection molding operation. Thismolding shrinkage of the resin results in generation of surfacedepressions which seem to occur most frequently among defects of themolded products.

FIG. 6 is a sectional view for explanation of the principle of thegeneration of such depressions. In FIG. 6, a reference numeral 5 denotesa depression, and a numeral 6 denotes a part of the resin which wascooled rapidly, which forms the outer portion of the molded product andwhich exhibits the same temperature change as that of the part of moltenresin 1 existing close to the surface of the metal mold shown in FIG. 4.A reference numeral 7 denotes another part of the resin which was cooledgradually and forms the inner portion (or the inner part of alarge-thickness portion) of the molded product and which exhibits thesame temperature change as that of the part of molten resin 2 existingin the inner part of the metal mold (or the layer of resin which formsthe inner portion of the molded product) shown in FIG. 4 as well. Thedepression 5 is caused to occur by such a phenomenon that the part ofresin 7 which was cooled gradually pulls down the part of resin 7 whichwas cooled rapidly.

In short, the depression referred to above is caused by the moldingshrinkage resulting from the very complicated behavior of the moltenresin as mentioned before and is accordingly considered to be a defectwhich has been very difficult to eliminate.

In order to prevent such depressions, there have hitherto been takenvarious measures such as, for example, a measure in which the molding isperformed under conditions such that the injection pressure is increasedand the temperature in the injection heating cylinder is lowered whilethe hold pressure is applied sufficiently; a measure in which thediameters of a sprue and a runner, particularly of a gate, areincreased; and a measure in which the molded product and the metal moldare designed, so that it will be difficult for a depression to occur.

However, it is hard to say that every prior measure is a satisfactoryand effective depression preventing measure because they are attendedwith technical difficulties and require annoying control means.

Incidentally, although not directly related to a depression preventingmeasure, a molding method is known in which a foaming agent is added tothe resin to form the molded products of spongy plastics such as, forexample, urethane foam and foam polyethylene. According to this method,the occurrence of depressions can be prevented due to increased pressureproduced when foaming. However, since such foaming conventionally takesplace even in the surface layer, the appearance of the molded product isnot as good as would be desired.

Further, what is called a "sandwich molding method" is also known inwhich a first resin, for forming the surface layer, is first introducedinto the mold under pressure and then another resin (combined with afoaming agent), for forming the inner part of the molded article, isintroduced into the mold under pressure. According to this method,however, the necessity of provision of two molding machines makes thestructure complicated, and the application of this method is limited tothe forming of the molded products having a large thickness.

Accordingly, an object of the present invention is to provide aninjection molding method for forming molded products of foamed plasticswhich is capable of preventing the occurrance of depressions and ofneatly finishing the appearance of the molded products by making use ofa novel measure that is entirely different from the conventionalmeasures in which an excessively high injection pressure and holdpressure are applied.

DISCLOSURE OF INVENTION

In order to achieve the above-described object, according to the presentinvention, there is provided an injection molding method comprising thesteps of: mixing a foaming agent, which begins foaming after being keptat a required temperature for a required time, into molten resin;pouring the molten resin mixed with the foaming agent into a metal mold;and causing only a part of the molten resin, which forms an inner partof a molded product and the cooling velocity of which is low, to effectfoaming, without causing another part of the molten resin, which is incontact with an inner surface of the metal mold, and which will form asurface of the molded product and the cooling velocity of which is high,to effect foaming.

In the step of cooling and solidification of the molten resin mixed withthe foaming agent and filled in the metal mold, the part of molten resinexisting close to the inner surface of the metal mold (or the layer ofresin for forming the surface of the molded product) 2 is cooled rapidlyas shown in FIG. 4 so that the foaming agent mixed in the molten resindoes not begin foaming at the temperature of this rapidly cooled resin.In consequence, the resin is solidified without causing the foamingagent to effect foaming in this part of the resin. As a result, it ispossible to obtain a molded product the surface or the appearance ofwhich is neat and even the strength of which is not deteriorated.

On the other hand, as shown in FIG. 4, the part of molten resin existingin the inner part of the metal mold (or the layer of resin which formsthe inner part of the molded product) 2 is cooled gradually so that thetemperature thereof is not rapidly decreased. Namely, the temperature ofthis part of resin is maintained higher than the temperature of the partof resin existing in the proximity of the surface of the metal mold. Thefoaming agent reacts at such high temperature to begin foaming. Owing tothe foaming pressure produced during such foaming, the molten resin inthe metal mold is expanded from inside. This expansion makes it possibleto prevent the occurrance of depressions which might otherwise be causeddue to shrinkage upon cooling of the resin.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional front view illustrating an outline of a firstembodiment of an injection molding machine for carrying out an injectionmolding method according to the present invention;

FIG. 2 is a sectional view illustrating an example of foaming agent usedin the injection molding method according to the present invention;

FIG. 3 is a sectional front view illustrating an outline of a secondembodiment of the injection molding machine for carrying out the presentinvention;

FIG. 4 is a graph showing the temperature change of molten resin in ametal mold;

FIG. 5 is a graph showing the changes in injection pressure and metalmold internal pressure after the injection;

FIG. 6 is a sectional view for explanation of the principle ofoccurrence of depression in a conventional injection molding method;

FIG. 7 is a sectional front view illustrating an outline of a thirdembodiment of the injection molding machine for carrying out the presentinvention;

FIG. 8 is a diagram showing the relation between the temperature changesin a heating cylinder and the metal mold;

FIG. 9 is a front view illustrating an outline of a fourth embodiment ofthe injection molding machine for carrying out present invention;

FIG. 10 is a diagram showing the relation between the progress oftemperature change of the resin and foaming agent during injectionfilling of the mold and cooling of the molded article;

FIG. 11 is a diagram showing curves of gas quantity generated as aresult of decomposition by heating depending upon temperature; and

FIG. 12 is a diagram showing curves of gas quantity generated as aresult of decomposition by heating depending upon time.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a front view illustrating an outline of a st embodiment of aninjection molding machine for carrying out present invention, in whichreference numeral 10 denotes a per for supplying resin; 11, an injectionheating cylinder which provided with a screw; 12, a fixed board; 13, afixed metal d; 14, a movable metal mold; 15, a movable board; 16, anejector 17, a sprue; 18, a runner; and 19, a metal mold cavity, theseknown elements constituting a molding machine body.

The injection molding machine applied to this embodiment constructed byattaching to the molding machine body described above a foaming agentmixing mechanism which is constituted by a per 20 for supplying foamingagent, a foaming agent extruding screw 21, a foaming agent mixingpassage 22, formed to extend vertically from the forward end portion ofthe screw 21 so as to be communicated with an insertion hole of theejector pin 16 passing through a portion of the movable metal mold 14, afoaming agent stirring air supply pipe 23, formed to be communicatedwith the passage 22, and a motor 24 for rotatively driving the screw 21.

For the foaming agent referred to above, a foaming agent of the typethat exhibits differences in its foaming reaction as a function ofdifferences in the rate of cooling of the molten resin filled in themetal mold cavity 19, that is, in such a manner that foaming is noteffected at the temperature of a part of the resin which is subjected toa high rate of cooling (or at the temperature of the rapidly cooledmolten resin part while foaming is effected at the temperature ofanother part of the resin which is subjected to a low rate of cooling(or at the temperature of the gradually cooled molten resin part 2), isused.

Specifically speaking, the following foaming agents are available, forexample: a foaming agent 32 of the type which is made in such a mannerthat, as shown in FIG. 2, foaming substance 31 (such asazodicarboxylicacidamide or sodium bicarbonate) is coated with a coatingagent 30 which becomes molten when heat is applied to maintain thetemperature at above a certain level for a certain time, that is, whenit is heated at a temperature above a certain specified temperature (orthe temperature of the part of resin the cooling velocity of which islow) for a specific period of time, and is then solidified to be shapedinto granules, or a foaming agent of the type in which the foamingreaction does not start until the foaming agent itself is heated to anelevated temperature (or the temperature of the part of resin thecooling velocity of which is low) (such as hydrazinedicarbonamide).

In the present embodiment, the foaming agent 32 which is coated as shownin FIG. 2 is used as the foaming agent.

Next, the injection molding operation according to the presentembodiment will be explained.

Referring to FIG. I, the fixed metal mold 13 and the movable metal mold14 are first clamped to each other, and the ejector pin 16 is then movedbackward to allow the foaming agent mixing passage 22 to be communicatedwith the sprue 17 and the runner 18. Thereafter, the motor 24 is drivento rotate the extruding screw 21. Then, the foaming agent extrudingscrew 21 is rotated so that the coated foaming agent 32 accommodated inthe hopper 20 is caused to drop into the movable metal mold 14 throughthe mixinq passage 22. In this case, in order to enable the coatedfoaming agent 32 to be fed smoothly from the passage 22 into the movablemetal mold 14, that is, in order to prevent stagnation of the foamingagent 32, air is introduced into the passage 22 through the stirring airsupply pipe 23 to stir the coated foaming agent 32. Subsequently, themolten resin in the heating cylinder 11 is forced under pressure intothe metal mold cavity 19. In this case, melting and injection of theresin and mixing of the coated foaming agent 32 into the molten resinare effected on the same axis.

Thereafter, the molten resin mixed with the coated foaming agent 32 andfilled in the metal mold cavity 19 is cooled and solidified inaccordance with the general method of the prior art and, subsequently,is released from the mold.

In the step of cooling and solidification mentioned above, since a partof the molten resin which exists close to the surface of the metal moldcavity 19 is cooled rapidly as indicated by a solid line 1 shown in FIG.4, the coating agent 30 which coats the foaming substance 31 as shown inFIG. 2 is not melted, with the result that the foaming agent 32 is notactivated but rather is with the resin without starting the foaming. Inconsequence, the appearance of the molded product can be kept neat, asin the case of the molding of non-foamed plastics, and, in addition,there is no fear that the strength of the molded product isdeteriorated. On the other hand, since another part of the molten resinwhich exists in the inner part of the metal mold cavity 19 is cooledgradually, as indicated by a broken line 2 shown in FIG. 4, thetemperature thereof is not rapidly decreased so that the coating agent30 shown in FIG. 2 becomes molten. This causes the foaming substance 31to react: so that the foaming agent 32 begins foaming. The foamingpressure produced during such foaming eliminates the occurrence of thedepression 5 which might otherwise be caused in molding of ordinarynon-foamed plastics as a result of withdrawal of the previouslysolidified portion due to the shrinkage of the inner part of the resinupon cooling, that is, a depression of the rapidly cooled resin part 6which appears in the surface of the molded product as shown in FIG. 6.

FIG. 3 is a sectional front view illustrating an outline of essentialportions of a second embodiment of the injection molding machine forcarrying out the present invention.

The injection molding machine used in the present embodiment has anarrangement of a molding machine body constituted by the same knownelements as shown in Figure that is, the injection heating cylinder 11provided with the screw, the fixed board 12, the fixed metal mold 13,the movable metal mold 1 4, the movable board 15, the ejector pin 16,the sprue 17, the runner 18 and the metal mold cavity 19, that hasattached thereto a foaming agent mixing mechanism which is constitutedby a hopper 40 for supplying the foaming agent, a foaming agentextruding screw 41, a foaming agent mixing piston 42 provided to extendvertically through a portion of the fixed metal mold 13 so as to becommunicated with the sprue 17, a foaming agent mixing passage 43 formedto extend from the forward end portion of the screw 41 so as to becommunicated with the foaming agent mixing piston 42, and a motor (notshown) for rotatively driving the screw 41

In the present embodiment, the foaming agent mixing piston 42 is firstmoved backward, the foaming agent 32 which is coated as shown in FIG. 2,for example, is then supplied from the hopper 40, and the molten resinis injected from the heating cylinder 11 while moving the piston 42forwardly, thereby mixing the coated foaming agent 32 into the moltenresin.

In other words, in the present embodiment, injection of the molten resinand supply of the foaming agent are effected on the same axis, and thecoated foaming agent 32 is directly mixed into the molten resin flowingalong this axis (or the sprue 17) from a direction perpendicular to thelatter.

In this way, the molten resin mixed with the coated foaming agent 32 isfilled into the metal mold cavity 19 and, after that, the same functionand effect as the case shown in FIG. 1 are exhibited.

FIG. 7 illustrates a third embodiment of the injection molding machinefor carrying out the present invention, in which a molding machine bodyis constituted by known elements including an injection heating cylinder102 provided with a screw 101, a nozzle 103, a fixed board 104, a fixedmetal mold 105, a movable metal mold 106, a sprue 107, a heater 108, arunner 109, a gate 110, and a cavity 111.

The molding machine body described above has attached thereto astructure comprising cooling blower air passage 112 through which air ismade to flow toward the heating cylinder 102 to absorb the heatliberated in the heating cylinder 102 by shearing action, and a hopper115 for supplying resin 113 which serves as a molding material and thefoaming agent 32.

Incidentally, as for the foaming agent 32 mentioned above, by making useof foaming agent 32 of the type that has a boiling point which is higherthan the melting point of the resin 113 so that the resin is molten butthe foaming agent 32 does not start the foaming reaction whileplasticization of the resin is being effected in the heating cylinder102, foaming agent 32 of the type that the temperature and retentiontime at that temperature required for beginning the foaming reaction areadjusted in accordance with the difference in the cooling rate of themolten resin filled in the cavity 111--thus, foaming is not effected atthe temperature of a part of the resin the cooling rate of which is high(or at the temperature of the rapidly cooled part of the resin), whilefoaming is effected at the temperature of another part of the resin thecooling rate of which is low (or at the temperature of the graduallycooled part of the resin), is used.

Specifically speaking, foaming agent 32 of the type that is made in sucha manner that, as shown in FIG. 2, a foaming substance 31 (such asazodicarboxylicacidamide or sodium bicarbonate) is coated with a coatingagent 30 which is molten when heat is applied to maintain thetemperature at above a certain level for a certain time, that is, whenit is heated at a temperature above a certain specified temperature(corresponding to the temperature of the part of the resin the coolingrate of which is low) for a specific period of time, and which is madeof a plastic material which is the same as the molding material but themelting point of which is made higher than that of the molding materialby a necessary amount by increasing its molecular weight, and is thensolidified and shaped into granules, is used.

Next, operation of the injection molding according to the presentembodiment will be explained. First, the fixed metal mold 105 and themovable metal mold 106 are clamped to each other, and the resin 113 andthe coated foaming agent 32 are then supplied from the hopper 115 so asto be mixed and kneaded with each other and dispersed in the heatingcylinder 102 due to rotation of the screw 101, and, thereafter, themolten resin is injected through the nozzle 103, thereby mixing thecoated foaming agent 32 into the molten resin.

Subsequently, the molten resin mixed with the coated foaming agent 32 ispoured under pressure into the cavity 111, and after that, the moltenresin mixed with the coated foaming agent 32 and filled in the cavity111 is cooled and solidified in accordance with the general method ofthe prior art and is thereafter released from the mold.

Next, FIG. 8 shows temperature changes in the heating cylinder and inthe metal mold.

It is noted that, in FIG. 8, a reference character J represents themelting point of the coating material, a reference character Krepresents the melting point of the molding material, and a referencecharacter L represents the foaming point of the foaming agent.

Further, in FIG. 8, reference characters 4', 5' and 6" represent a rangeof temperature change exhibited within the screw 1, a range oftemperature change exhibited while passing through the nozzle 3, a rangeof temperature change exhibited while passing through the sprue 7, arange of temperature change exhibited while passing through the runner9, a range of temperature change exhibited while passing through thegate 10, and a range of temperature change exhibited while the moldingmaterial is being filled into the mold and in the process of shrinkage,respectively.

In addition, a solid line 7' indicates the temperature change of thepart of the molding material exiting close to the skin of the metalmold, a broken line 8' indicates the temperature change of the foamingsubstance inside the coating agent in terms of the threshold of foamingreaction, and the one-dot chain line 9' indicates the temperature changeof the part of the molding material which forms the inner part of themolded product.

In the step of cooling and solidification mentioned above, although theresin 113 is molten within the screw 101 provided in the heatingcylinder 102 because the melting point of the resin 113 serving as themolding material is represented by K, the coating agent 30 and thefoaming substance 31 do not reach their reaction temperature, as seen inthe range 1', since air is made to flow through the blower air passages112 to absorb the heat generated by the shearing. By injecting, into themetal mold, the molten resin and the foaming agent which have been mixedand kneaded with each other, the temperature is increased while passingthrough the nozzle 103 as seen in the range 2', the temperature changeexhibited while passing through the sprue 107 is as shown in the range3', the temperature change exhibited while passing through the runner109 is as shown in the range 4', the temperature change exhibited whilepassing through the gate 10 is as shown in the range 5', and thetemperature change exhibited while filling the product molding portionin the metal mold and after the completion of filling into the productmolding portion is as shown in the range 6'.

More specifically, the part of the resin existing close to the metalmold exhibits the temperature change indicated by the solid line 7', thegradually cooled part of the resin existing in the inner part of theproduct molding portion in the metal mold, which acts as the cause forthe depression, exhibits the temperature change as indicated by theone-dot chain line 9', and the threshold of thermal work done thatcauses the coating agent 30 of the foaming agent 32 to become molten tostart the reaction is exhibited as the temperature change indicated bythe broken line 8'. Accordingly, the foaming reaction takes place attemperatures indicated by the one-dot chain line 9' above the brokenline 8', while no foaming reaction takes place at temperatures indicatedby the solid line 7' below the broken line 8'. Namely, the thermal workdone achieves temperatures above the foaming point L of the foamingagent in the part of the resin existing close to the skin of the metalmold which corresponds to an area which is surrounded by points A', Pand B' so that no foaming is effected, while the thermal work doneachieves temperatures above the foaming point L of the foaming agent inthat part of the resin existing in the inner part of the metal moldwhich corresponds to an area, which is surrounded by the points A', Pand C', so that foaming is effected.

In this way, since the molding can be effected in the non-foamed stateduring and immediately after the filling of the resin into the productmolding portion in the metal mold, it is possible to form productshaving a complicated configuration, such as a small-thickness portion orprojection and, furthermore, it is possible to obtain molded productshaving a neat appearance.

In addition, as measures for preventing the occurrence of depressionwhich might otherwise be caused due to the phenomenon that the graduallycooled part of the resin pulls down the rapidly cooled part of the resinafter the gate is sealed, the gate 1 10 is enlarged and the injectionpressure is increased, and accordingly, it is possible to lessen fin andcamber as well as depression.

Furthermore, since foaming takes place only locally, it is possible toremarkably reduce the cycle time in comparison with the case of ordinaryfoaming

Incidentally, in the present embodiment, while the molding material 113and the coated foaming agent 32 have been described as being put intothe hopper 115 separately from each other, however, the foaming agent 32may be mixed into the molding material 113.

FIG. 9 is a schematic view illustrating a fourth embodiment of aninjection molding machine for carrying out the present invention, inwhich a reference numeral 201 denotes an injection heating cylinder;202, a screw; 203, a nozzle; 204, a sprue; 205, a runner; 206, a gate;and 207, a product molding portion of the metal mold, these elementsconstituting all together a molding machine body.

A hopper 208 for supplying resin serving as molding material and foamingagent is attached to the molding machine body described above.

As for the foaming agent referred to above, foaming agent of the typethat the temperature and holding time at that temperature required forstarting the foaming reaction are adjusted in accordance with thedifference in the cooling rate of the molten resin filled in the productmolding portion 207 of the metal mold, that is, in such a manner that nofoaming takes place at the temperature of that part of the resin whosecooling rate is high (corresponding to the temperature of the rapidlycooled part of the resin), while foaming takes place at the temperatureof another part of the resin whose cooling rate is low (corresponding tothe temperature of the gradually cooled part of the resin), is used.

Specifically speaking, foaming agent of the type which does not startits foaming reaction as long as the foaming agent itself is not heatedto an elevated temperature (or the temperature of the part of the resinthe cooling rate of which is low) (such as hydrazinedicarbonamide thedecomposition temperature of which is 245° C. or azodicarbonamide thedecomposition temperature of which is 205° C.), is used, for example.

Next, the operation of the injection molding according to thisembodiment will be explained.

Referring to FIG. 9, the product molding portion of the metal mold isfirst clamped and, then, the molding material (pellets) and the foamingagent are sent from the hopper 208 so as to be mixed and kneaded witheach other in the heating cylinder 201 due to the rotation of the screw202, and thereafter, the molten resin and the foaming agent are injectedthrough the nozzle 203.

Then, the molten resin mixed with the foaming agent is poured underpressure into a cavity which is not shown and, after that, the moltenresin mixed with the foaming agent and filled in the cavity, which isnot shown, is cooled and solidified in accordance with the generalmethod of the prior art and is thereafter released from the mold.

Incidentally, in FIG. 9, reference characters 1', 2", 3', 4', 5', 6',7'and 8'represent the supply area of the screw, the compression area ofthe screw, the measuring area of the screw, the nozzle area, the spruearea, the runner area acting also as a second sprue area, the gate area,and an area of the product molding portion of the metal mold,respectively.

Further, reference characters 1', 2', 3', 4', 5', 6', 7', and 8' shownin FIG. 10 represent the progress of temperature changes of the resinand the foaming agent in correspondence with the areas represented bythe same reference characters, respectively, in FIG. 9. In addition,reference characters 9', K', and L' represent a foaming range, thedecomposition temperature of the foaming agent, and the time required tobegin foaming, respectively.

FIG. 11 shows curves of gas quantity generated as a result ofdecomposition by heating depending upon the temperature, in whichreference characters A, B and C refer to the differences caused bychanging the chemical constitution itself, reference characters A₁, A₂and A₄ mean the difference due to fine adjustment made by mixing apromoter of the urea group, for example. Likewise, reference charactersB₁, B₂, B₃ and C₁, C₂, C₃ refer respectively to the differences due tomixing of the promoter. In addition, a reference character M representsthe point at which complete foaming is achieved.

FIG. 12 shows curves of gas quantity generated as a result of thedecomposition by heating of different foaming agents X, Y and Zdepending upon time. It is understood from FIG. 12 that the timerequired for beginning foaming is adjustable.

In the step of cooling and solidification mentioned before, since thetemperature in the screw 202 of the heating cylinder 201 is lower thanthe decomposition temperature K' of the foaming agent, as shown in thesecond ranges 1', 2'and 3', no foaming takes place. When injected intothe metal mold, the temperatures of the resin and the foaming agent areincreased due to exothermic heat generated by shearing. However, sinceholding time is required to start foaming as shown in FIG. 12, foamingdoes not take place until immediately after the product molding portion207 is filled up but takes place after a lapse of time L' which isrequired for starting foaming in the gradually cooled region within themetal mold only in correspondence with the foaming range 9' shown inFIG. 10.

More specifically, since the thin portion close to the surface of themetal mold is cooled rapidly, as indicated by a broken line shown inFIG. 10, the foaming agent does not come to be molten but rather issolidified in a body with the resin without effecting any foamingreaction. In consequence, the appearance of the molded product can bekept neat, as is the case of the molding of non-foamed plastics and, inaddition, there is no fear that the strength of the molded product isdeteriorated. On the other hand, in the gradually cooled region of largethickness, since the temperature does not decrease because the coolingis effected gradually, the foaming agent is molten to start foaming asindicated by the foaming range 9' shown in FIG. 10. Owing to the foamingpressure produced during such foaming, it is possible not only to makeit difficult for a depression to occur, which might otherwise be causedin the molding of ordinary non-foamed plastics as a result of withdrawalof the previously solidified portion due to the shrinkage of the innerpart of the resin upon cooling, but also to reduce the pressure neededto fill the metal mold, with the result that it is possible to decreasefin and camber.

INDUSTRIAL AVAILABILITY

According to the present invention, it is possible to obtain moldedproducts of foamed plastics the appearance of which is neat and thestrength of which is not deteriorated. Further, since it is possible toprevent the occurrence of unwanted depressions without applyingexcessively high injection pressure (or hold pressure), which has beenapplied in the prior art, there is no need to fill the resin in such amanner that an excessively high internal pressure is produced in themold, with the result that the occurrence of fin can be reduced. Inconsequence, the present invention is favorable to the molding ofprecision parts which abound in small parts, and, at the same time, theinvention has the advantage of wide applicability from small machines tolarge machines. Accordingly, the present invention is applicable to theinjection molding, seal molding and the like for forming products otherthan transparent products and every kind of small-thickness product,such as containers.

What is claimed is:
 1. An injection molding method comprising the stepsof:(a) mixing a foaming agent, which begins foaming after being kept ata required temperature for a required time, into molten resin; (b)filling a mold with a sufficient amount of said molten resin mixed withsaid foaming agent to produce a molded product conforming to said mold;(c) cooling a first portion of said mixture, which forms an inner partof said molded product, at a low rate sufficient to cause foaming ofsaid first mixture portion; and (d) cooling a second portion of saidmixture, which is in contact with an inner surface of said mold, at ahigh rate sufficient to substantially prohibit foaming of said secondmixture portion, thereby substantially eliminating undesired surfacedepressions and forming a substantially neat surface of the moldedproduct, wherein said foaming agent has a structure in which a substancecausing said foaming is coated with a coating agent and said foamingagent is mixed into said molten resin immediately before said moltenresin is filled into said mold.
 2. An injection molding method accordingto claim 1, wherein said substance causing said foaming is composed ofazodicarboxylicacidamide.
 3. An injection molding method according toclaim 1, wherein said substance causing said foaming is composed ofsodium bicarbonate.
 4. An injection molding method according to claim 1,wherein said substance causing said foaming is composed ofhydrazinedicarbonamide.